This invention relates to a printer, particularly a color printer, operating on the principle of thermal ink transfer, such that ink is thermally transferred from ink transfer sheet to ink recipient sheet. More particularly, this invention concerns means in such a printer for automatically readjusting the position of the recipient sheet with a view to the enhancement of the registration or alignment of a plurality (normally three or four) of monochrome images of each color reproduction.
The thermal ink transfer printer has won extensive commercial acceptance as a compact color printing device. Among the primary reasons for this popularity are the simplicity in construction, fast printing speed, and ease of handling of the printer, as well as the high resolution and the multiple tone gradations of the printings obtained.
Basically, in the printer of this type, each recipient sheet from an insertable cassette is printed upon while being pressed against a platen roll by the thermal printing head via an elongate strip of transfer sheet. The printing head has a plurality of electric heater elements which are aligned lengthwise of the platen roll. The electric current fed to the heater elements is controlled for printing dots on the recipient sheet line by line with the incremental rotation of the platen roll.
For color printing, a color transfer strip is employed which bears on its different longitudinal sections the inks of the three primary colors, yellow, magenta and cyan. A black ink may also be used to add detail and contrast to the printed reproduction, as is well known in the printing art. The ink of a first preselected color is first transferred from the color transfer strip to the recipient sheet while the latter is traveling forwardly past the platen roll in superposition with the required ink section of the color transfer strip. Then the recipient sheet is fed back by the platen roll in coaction with a pinch roller pressing the sheet against the roll. Then the recipient sheet is fed forward for the transfer of the ink of a second preselected color from the transfer strip to the recipient sheet. The same procedure is repeated on the same recipient sheet until the three or four monochrome images of the desired color reproduction are all formed in register on the recipient sheet.
There has been a problem left unsolved in the color printer of this type with regard to the registration of the set of monochrome images of each color reproduction. The problem arises from the fact that the recipient sheet must be fed back and forth past the platen roll several times for each color reproduction. The pinch roller has so far been held pressing the recipient sheet against the platen roll during all such repeated reciprocation of the recipient sheet. Moreover, the recipient sheet has been fed back at a speed several times higher than that during printing, with a view to the reduction of the total time of printing for each color reproduction.
For all these reasons the recipient sheet has tended to develop undulations, particularly with load fluctuations on the platen drive motor. Further the recipient sheet has been easy to become displaced with respect to the platen roll because of unavoidable variations in the cylindricity and the dimensional and positional accuracy of the platen and pinch roller. The displacement of the recipient has also been liable to occur from variable degrees of frictional resistance imposed thereon with respect to the transverse direction of the recipient sheet.
Thus, conventionally, the set of monochrome images of each color reproduction have sometimes been printed out of register with one another. The degree of color registration has been subject to change from one reproduction to another, to the impairment of the reliability of the color ink transfer printer.
It might be contemplated, for example, to manufacture the platen, pinch roller and other associated means to closer dimensional tolerances, and to assemble them with greater positional accuracy, than heretofore. This solution would be impractical because such parts must be made of elastic material and so are susceptible to deformation. The color printer itself would also become much more difficult of manufacture and expensive in construction.